1. Field of the Invention
This invention relates to a method of selectively plating an area of a substrate surface and more particularly, to a method including the use of a dielectric member contact masking an anode.
2. Description of the Prior Art
Over the years, many systems and methods were developed for plating patterns of metal on metal or other substrates. Some techniques include masking of the substrate to produce the desired pattern or confining the electroplating electrolyte to discrete portions of the substrate. The elimination of this particular masking or confining requirement would contribute greatly to the advancement of the art since such masking increases the complexity and cost of the process. Such an elimination of masks is especially desirable in the manufacture of integrated circuits where selective or spot plating is essential in order to eliminate the waste of precious metals such as gold.
In the manufacture of integrated circuits, leads of each of a plurality of integrated circuit chips are bonded individually to gold conductive patterns formed on insulating ceramic substrates. In order to connect these patterns to other circuits, leads are connected to these patterns and are often formed from a lead frame stamped from a sheet of conductive metal such as nickel, copper or the like. The lead frame has a separate group of leads for each conductive pattern of each substrate, and these leads are bonded to the substrate. Each group has the outer ends of each lead interconnected to carrier strips and their opposed inner free ends bonded to the conductive pattern of each substrate. The leads are also interconnected intermediate their ends by relatively narrow support strips. The carrier strips have outer portions perforated for feeding them through various treating operations. Both the carrier and support strips are severed to separate the individual leads prior to the completion of the ultimate integrated circuits.
In the prior art techniques, a layer of gold has been often formed over the entire lead frame by electroless or electroplating. The purpose of such a gold layer is to improve any bonds made to the lead frame. In other prior art techniques, a non-uniform layer of gold has been formed over the entire lead frame with the greatest thickness being concentrated at those portions of the frame where the bonding of an integrated circuit chip or other semiconductive device is to take place (see Rackus et al., U.S. Pat. No. 3,692,638). Since the carrier and support strips are ultimately trimmed away it is desirable to have no gold on them. Since the substrates are bonded to the inner free ends of the leads, it is desirable to have a gold layer thereon to improve bondability. Since no substrates are bonded to the remainder of the leads, a gold layer thereon is not necessary. The absence of gold on these strips eliminates expensive and time-consuming reclamation processes to recover the gold from these strips. The absence of gold everywhere except where substrate bonding is to take place can result in a very desirable efficient use and substantial savings of gold. It is desirable to bring about selective or spot plating without the use of substrate (lead frame) masks. The elimination of such masks often simplifies the treating process and eliminates any mask replacement or cleaning steps.
Selective plating enhancement techniques are known in the art in which discrete areas of a surface are electroplated at a much greater or enhanced rate than other areas which are simultaneously being plated. U.S. Pat. No. 2,828,255; British Pat. No. 775,359; German Pat. No. 577,747; and German Pat. No. 850,972 contain typical examples of such techniques. However, as shown in the above patents, the surface being plated is immersed in electroplating electrolyte and is consequently essentially immediately blanket electroplated rather than being selectively electroplated. It is desirable to obtain selective electroplating on discrete areas of a surface in a practical, workable thickness, e.g., typically 100.mu. inches, without electroplating on areas adjacent thereto. This is a requirement which is not readily feasible by immersion of the entire surface (unmasked) in the electroplating electrolyte.